Tissue oxygenation is often used as an indicator of perfusion status in patients experiencing undifferentiated shock. High risk patients who receive continuous monitoring of tissue oxygenation from the trauma bay through X-ray and CT imaging as well as other procedures have been shown to receive effective interventions sooner, resulting in significant reductions in ICU admission, length of stay, morbidity, and mortality.
Near Infrared Spectroscopy (NIRS) is one technique used for non-invasively measuring tissue oxygenation. To measure tissue oxygenation, NIRS systems use complex mathematical algorithms that relate light attenuation, measured at multiple wavelengths, to the concentration of different hemoglobin forms, such as oxyhemoglobin (HbO2) and deoxyhemoglobin (HHb). The concentration of these hemoglobin forms or their calculated oxygen saturation ratio ([HbO2/[HbO2+HHb]), defined as saturated oxygen level or “StO2,” provides an indication of how much oxygen is available to the tissue. A clinician may use these tissue oxygen measurements to evaluate a patient's health status and make treatment decisions.
NIRS devices employ a light source that illuminates tissue at specific wavelengths of light, typically between 650 nm to 1000 nm, and at least one photodetector that measures the amount of light exiting the tissue within a given area. An example of a NIRS spectrometer is described, for example, in U.S. Pat. No. 7,239,901, the contents of which are incorporated herein by reference in their entirety for all purposes.
During operation, the amount of light exiting the tissue is compared to the amount of light emitted into the tissue in order to measure the amount of light lost in the tissue, which is defined as light attenuation. In tissue, light attenuation occurs from absorption and scattering events. Light absorbing molecules, called chromophores, convert light to heat energy thus reducing the amount of detected light. Light scattering molecules, such as tissue cells and organelles, refract light thereby changing the direction and hence path length that the light travels. Although some scattering is required to direct light to the detector in a reflectance probe configuration, the scattering effect on the light path limits and reduces the amount of light that eventually exits the tissue where the photodetector is placed. A reduction in detected light, either from absorption or scattering events, therefore increases the amount of light attenuation measured with a NIRS spectrometer.